How the class loader works in Java with a shaded jar

Question

My experience with Java class loading is limited. With tools like Maven I have a rusty understanding of how they resolve dependency versions. But I've hit a problem that's making me question how Java loads classes.

My scenario, I have a dependency on version 30.1.1-jre of com.google.guava . I also have a shaded jar which has a dependency on Guava 18.0.

In my application I end up seeing the following exception

java.lang.IncompatibleClassChangeError: Class com.google.common.base.Suppliers$MemoizingSupplier does not implement the requested interface java.util.function.Supplier

which I cannot reproduce locally. Based on https://github.com/crabhi/celery-java/issues/9 it sounds like this error is produced when an older version of Guava is on the classpath.

Checking the classes in the war I see

WEB-INF/lib/java-driver-shaded-guava-25.1-jre-graal-sub-1.jar.d/com/datastax/oss/driver/shaded/guava/common/base/Suppliers$MemoizingSupplier.class

WEB-INF/lib/nautilus-es2-library-2.3.4.jar.d/com/google/common/base/Suppliers$MemoizingSupplier.class

WEB-INF/lib/guava-30.1.1-jre.jar.d/com/google/common/base/Suppliers$MemoizingSupplier.class

This makes me think the shaded jars are causing a problem.

Is that possible? Are there any articles explaining how classes are loaded when shaded jars enter the picture?

Answer 1

You have two copies of com.google.common.base.Suppliers.MemoizingSupplier on the classpath. Only one can be loaded, and in your case, it's the older version.

You shouldn't have multiple classes with the same name available to one classloader. What is nautilus-es2-library-2.3.4.jar and why does it bundle Guava rather than expressing it as a transitive dependency?